If one thinks of the Stone Giants as metaphorical, then they are already part of the storm. If they're literal, then they're an additional threat.

...first to the Great Goblin and later in reference to sealing up the cavern in the pass eliminate the notion that they were meant to be metaphorical in nature. 'There are older and fouler things than Orcs in the deep places of the world.' - Gandalf the Grey, The Fellowship of the Ring